Embryonic development and cellular differentiation are considered unidirectional pathways because cells undergo a progressive loss of developmental potency during cell fate specification. Two categories of pluripotent stem cells are known to date: embryonic stem cells and embryonic germ cells. Embryonic stem cells are pluripotent stem cells that are derived directly from an embryo. Embryonic germ cells are pluripotent stem cells that are derived directly from the fetal tissue of aborted fetuses. For purposes of simplicity, embryonic stem cells and embryonic germ cells will be collectively referred to as “ES” cells herein.
The success of somatic cell nuclear transfer (SCNT) experiments in mammalian species provided proof that the epigenetic state of adult differentiated cells is not fixed but remains pliable for reprogramming by factors present in the oocyte cytoplasm (Byrne et al., 2007; Jaenisch and Young, 2008; Wakayama and Yanagimachi, 2001). However, the inefficiency and ethical concerns associated with attempting to clone human somatic cells have spurred the field to search for alternative methods to achieve nuclear reprogramming without using oocytes (Jaenisch and Young, 2008). Indeed, fusion of somatic cells to embryonic carcinoma cells or embryonic stem (ES) cells results in epigenetic resetting of the somatic genome but involves the generation of 4N pluripotent cells, limiting the potential therapeutic use of such cells (Cowan et al., 2005; Tada et al., 2001).
Nevertheless, the reprogramming of somatic cells by fusion with ES cells suggested that ES cells, similar to the oocyte cytoplasm, contain factors that can induce nuclear reprogramming. An important breakthrough was achieved by Yamanaka and colleagues, who succeeded in directly reprogramming fibroblasts into induced pluripotent stem (iPS) cells by transduction of the four transcription factors Oct4, Sox2, Klf4 and c-Myc (Takahashi and Yamanaka, 2006). Although the initially obtained iPS cells were not normal, several groups have since advanced the direct reprogramming technique by generating iPS cells that are epigenetically and developmentally indistinguishable from embryo-derived ES cells (Maherali, 2007; Meissner et al., 2007; Okita et al., 2007; Wernig et al., 2007). Moreover, transgenic expression of c-Myc was found to be dispensable for reprogramming, though it accelerated and enhanced the efficiency of reprogramming (Nakagawa et al., 2008; Wernig et al., 2008). Finally, it has also been shown that human iPS cells can be generated by transduction of defined factors into somatic cells (Park et al., 2008; Takahashi et al., 2007; Yu et al., 2007).
Despite the work that has been done to date, it remains unknown whether terminally differentiated cells can be reprogrammed to pluripotency with defined factors, or whether only less differentiated cells such as somatic stem cells can undergo nuclear reprogramming to pluripotency. Moreover, it is unclear whether progressive differentiation of the donor cells affects the efficiency of in vitro reprogramming.